SDK: span processor interface and simple span processor. (#117)

* SDK: SpanProcessor Interface.

* add simple span processor.

* rename span processor.

* fix logic to export or process span data.

sdk/trace/span_processor.go

@@ -0,0 +1,74 @@
+// Copyright 2019, OpenTelemetry Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package trace
+
+import (
+	"sync"
+	"sync/atomic"
+)
+
+// SpanProcessor is interface to add hooks to start and end method invocations.
+type SpanProcessor interface {
+
+	// OnStart method is invoked when span is started. It is a synchronous call
+	// and hence should not block.
+	OnStart(sd *SpanData)
+
+	// OnEnd method is invoked when span is finished. It is a synchronous call
+	// and hence should not block.
+	OnEnd(sd *SpanData)
+
+	// Shutdown is invoked when SDK shutsdown. Use this call to cleanup any processor
+	// data. No calls to OnStart and OnEnd method is invoked after Shutdown call is
+	// made. It should not be blocked indefinitely.
+	Shutdown()
+}
+
+type spanProcessorMap map[SpanProcessor]struct{}
+
+var (
+	mu             sync.Mutex
+	spanProcessors atomic.Value
+)
+
+// RegisterSpanProcessor adds to the list of SpanProcessors that will receive sampled
+// trace spans.
+func RegisterSpanProcessor(e SpanProcessor) {
+	mu.Lock()
+	defer mu.Unlock()
+	new := make(spanProcessorMap)
+	if old, ok := spanProcessors.Load().(spanProcessorMap); ok {
+		for k, v := range old {
+			new[k] = v
+		}
+	}
+	new[e] = struct{}{}
+	spanProcessors.Store(new)
+}
+
+// UnregisterSpanProcessor removes from the list of SpanProcessors the SpanProcessor that was
+// registered with the given name.
+func UnregisterSpanProcessor(e SpanProcessor) {
+	mu.Lock()
+	defer mu.Unlock()
+	new := make(spanProcessorMap)
+	if old, ok := spanProcessors.Load().(spanProcessorMap); ok {
+		for k, v := range old {
+			new[k] = v
+		}
+	}
+	delete(new, e)
+	spanProcessors.Store(new)
+}